1. Technical Field
The present invention relates in general to a tape transport servo system and method for a computer drive and in particular to a tape transport servo system and method that eliminates the use of tape deck encoders and/or tape-path tachometers for controlling tape velocity and position. The present invention relates to a tape transport servo system and method that derives tape velocity from a primary velocity signal, such as from a servo track pre-recorded on the tape, and that also derives tape velocity from a secondary velocity signal, such as from pulse-width-modulation (PWM) signals driving two reel motors.
2. Description of the Related Art
Tape drives for computer systems have been in existence in the computer field. One type of tape drive, such as the Sapphire media tape drive, involves two reels of tape wherein one of the two reels is a supply reel and the other of the two reels is a take-up reel. The tape on the reels is typically a half inch (0.5xe2x80x3) wide. The reels are housed in a tape drive cartridge, and this type of tape drive typically has a 5.25xe2x80x3 form factor wherein the form factor (i.e. physical size of the tape drive or cartridge) is 6xe2x80x3 across, 3xe2x80x3 high, and 12xe2x80x3 deep. The supply reel is driven by a supply reel motor while the take-up reel is driven by a take-up reel motor. Control algorithms are used to control the speed of these two motors.
For these types of tape drives or tape transports, the position and velocity of the tape need to be controlled. The control algorithms require the determination of the position and velocity of the tape in order to properly control the speed of the reel motors. Tachometers and encoders are presently used to calculate the position and velocity of the tape. The tachometer and encoder are mounted near the motor, and they put out pulses to a monitoring logic card having a set of counters. The monitoring card has a processor and memory. The processor is a digital signal processor (DSP) inside the tape drive. The processor computes velocity and position, and the processor computes how much tape is on each reel. The processor uses all of the information to compute the electrical current driving the motor. The two tape reels, tape, and two motors that drive the reels are considered a plant. The plant receives multiple inputs and sends multiple outputs. For example, inputs to the plant are continuous currents to the motor for controlling the motor. The outputs from the plant are velocity, tape tension, and position. Tachometers have been used to count a wheel that rotates with a respective motor or a wheel that is driven by the tape. A tachometer is an optical device, which generates a pulse waveform having 512 pulses per turn of the motor. If faster pulses are generated, then the faster the motor is turning. An encoder puts out a pulse train having a frequency proportional to the frequency of the motor.
In some present IBM tape drives or products, two tachometers are used, that is, a tachometer is mounted and coupled to each of the two motors. Alternatively, a tachometer is mounted on a wheel that is in the tape path. The tachometer puts out a pulse form in proportion to the speed of the tape or motor, such as one pulse per revolution of the motor. Index pulses are needed from each reel. A determination of the radius of the tape on each reel is required to determine speed of the motor and velocity of the reel. The radius of the tape on a reel is determined from the tachometer and the pulse per turn for each reel and by using the encoder. However, since the tape drive is a 5.25xe2x80x3 form factor, physical and mechanical space becomes an issue. Digital tachometers and encoders take up space within the tape cartridge. Therefore, it is advantageous and desirable to eliminate the use of tachometers and encoders in these tape drives due to physical and mechanical space concerns.
Linear tape open (LTO) is a standard that presently exists for tapes and tape drives. The LTO standard for tape and tape drives is incorporated by reference herein. FIG. 1 illustrates a conventional format for a LTO tape 48, and FIG. 2 illustrates the manner of how a read/write head 46 reads the servo tracks 49 and writes data to the LTO tape 48. In FIG. 1, the LTO tape generally comprises five (5) servo tracks 49 that make up four (4) servo bands 47. Each servo track comprises a number of bursts 96 or pulses. The timing measurement of these bursts 96, which may be for example in a 5-5-4-4 pattern, are used to generate a primary velocity signal. Referring to FIG. 2, a data band 45 exists between each two (2) adjacent servo tracks 49, and each data band comprises eight data tracks 94 where data is written thereto. A read/write head 46 spans two servo tracks 49 and one data band 45 at any one time. The two servo read heads 90 of the read/write head 46 each respectively read one of the two servo tracks 49 while the write head 92 of the read/write head 46 writes to the data tracks 49 of the data band 45. The read/write head 46 moves from one servo band 47 to another servo band 47 when reading/writing to tape 48.
Thus, times exist when the head(s) 46 does/do not read the signals from the servo tracks 49 of the LTO tape 48, such as starting and stopping of the tape 48 and reels 41 and 43 or when the tape 48 and reels 41 and 43 are below some minimum speed or above some maximum speed or when the servo head(s) 46 is/are indexing laterally between the servo tracks, or servo bands which are spaced laterally across the tape width. Therefore, a secondary or alternative manner for deriving velocity and position of the tape 48 is also needed and desired, especially when the primary and direct manner of deriving the tape velocity and position cannot be used.
It is therefore advantageous and desirable to provide a tape transport servo system and method that do not make use of tachometers and/or encoders to derive tape velocity and position. It is also advantageous and desirable to provide a tape transport servo system and method that are able to derive velocity and position of the tape from a primary or direct manner and also from a secondary or alternative manner, especially when the primary and direct manner cannot be used.
It is therefore one object of the present invention to provide a tape transport servo system and method that do not make use of tachometers and/or encoders to derive tape velocity and position.
It is another object of the present invention is to provide a tape transport servo system and method that are able to derive velocity and position of the tape from a primary and direct manner and also a secondary or alternative manner, especially when the primary or direct manner cannot be used.
It is another object of the present invention to provide a tape transport servo system and method that rely on deriving the velocity from both a primary velocity source and signal and a secondary velocity source and signal.
The foregoing objects are achieved as is now described. A tape transport servo system and method that do not make use of tachometers and/or encoders to derive tape velocity and position are provided. The tape transport servo system and method derive velocity and position of the tape from a primary and direct manner and also a secondary or alternative manner, especially when the primary or direct manner cannot be used. The tape transport servo system and method controls a tape drive and moves a tape between one reel driven by one motor to another reel driven by another motor. The tape is read by a recording head composed of data readers, data writers and a dedicated set of servo read elements. A position and a velocity of the tape are determined from a primary velocity source when a servo track recorded on the tape is being read by the head. The position and the velocity of a servo track recorded on the tape are determined from a secondary velocity source when the tape is not being read by the head. The velocity of the tape is compared to a desired programmed reference velocity for the tape. The velocity of the tape is driven to the desired programmed reference velocity if the velocity is not equal to the desired velocity. The velocity of the tape is controlled by regulating the velocity and the driving currents of the motors. The tape transport servo system comprises a digital signal processor and a servo motor control system. The servo motor control system comprises a servo logic system, sensors, PWM motor drivers, power amplifiers, DACs, counters, computation systems, and a timing-based servo pattern monitoring system.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.